In a mobile communication system, for example GSM (Global System for Mobile communications) one or more radio access networks (RAN) are connected to a core network (CN). For GSM networks the RAN consists of a BSS (Base Station Subsystem) comprising a number of base station controllers (BSC), each controlling a number of base tranceiver stations (BTS) and serving the mobile stations connected to a number of BTSs. Each BSS (BSC) is connected to a mobile switching center (MSC) over the A-interface and each MSC can serve one or more BSCs. It should be clear that only those parts of the mobile communication system that are of interest for the present invention are discussed. Each BSC in a GSM network is able to control a plurality of radio cells and each BSC interworks with a plurality of BTSs via respective interfaces. Each BTS is in turn responsible for transmission and reception of radio signals over an interface in one or more cells. The BTSs also carry a number of connections or calls between mobile stations (MS) and respective BSCs, each BTS being equipped with a number of transceivers (TRX). In a so called split architecture the MSC is divided into a server node, here also called an MSC server, and a gateway node. The gateway node is connected to the BSS, or a BSC, and handles the user plane traffic of the A-interface, CS (circuit switched) payload, e.g. a number of speech channels. However, also the server node is connected to BSS (BSC), handling the control signalling of the A-interface using the BSSMAP/DTAP protocols. This is described in 3GPP TS 48.008. The server node and the gateway node communicate over the H.248 protocol. If a 3G access technology (3G, Third Generation) is used, an RNC (Radio Network Controller) provides for the controlling functionality corresponding to that of a BSC. In 3G the BTSs correspond to NodeBs. A common name for GSM BTSs and 3G nodeBs are radio base stations (RBSs). GSM as well as 3G (e.g. UMTS) networks are standardized by 3GPP, Third Generation Partnership Project.
In for example a BSS, all connections for circuit switched services are via the A-interface. For a speech call between two subscribers using radio resources handled by one and the same BSC throughout the call, connections are established for both subscribers from that BSC to the MSC. Particularly, in a split architecture, the user traffic flows of the connections are interconnected in the gateway node. A call or a connection is here seen as consisting of two call legs, one from the first subscriber towards the core network and one between the second subscriber and the core network. A call is set up via the MSC, (in case of split architecture the user plane is handled by the gateway node) since the BSS is not aware of the two call legs belonging to the same call or conversation. This is very disadvantageous in that resources are wasted, transmission costs will be high, the speech call quality will be degraded. The speech call quality is degraded for two reasons; first, increased propagation delay (e.g. if using satellite links), second, multiple coding/decoding of speech (e.g. A-interface is defined as using decoded speech (PCM (Pulse Code Modulation) format). This means that if the two MSs are served by one and the same BSC, payload will be routed between BSC and the gateway node and hardware as well as software resources will be used both in BSC and the gateway node. Particularly two transcoders will be connected in the BSC which will impact the voice quality. To solve some of these problems it has been suggested to introduce logic in the server node to determine if two call legs are controlled by the same radio network controller, particularly BSC, and that knowledge is used to optimize the switching path.
U.S. Pat. No. 6,958,983 shows a solution according to which new messages are introduced on the A-interface. One new message is intended to inform the BSS that call legs identified by the circuit identity codes (CIC) included in the message can be locally connected in the BSS to provide optimal routing of calls. Another message is provided having the purpose to inform the BSS that the original call legs identified by the CICS included in the message should be restored and not anymore locally connected in the BSS. The access network here is an IP based BSS and the first message, join CIC message, includes information elements with information about which CICS and signalling connections belong to a single conversation. This means that the associated call can be switched in an optimal way. The join CIC message is sent from MSC to BSS to inform the BSS that the CICS included in the message can be connected in the BSC. This means that the decision making is done in the server node. The solution is centralized and e.g. not easily adaptable to MSC pooling and shared architectures.